Characterization of extracellular micronuclei

Microglia, resident immune cells in the central nervous system, undergo morphological and functional changes in response to signals from the local environment and mature into various homeostatic states. However, niche signals underlying microglial differentiation and maturation remain unknown. Here, we show that neuronal micronuclei transfer to microglia, which is followed by changing microglial characteristics during the postnatal period. Neurons passing through a dense region of the developing neocortex give rise to micronuclei and release them into the extracellular space, before being incorporated into microglia and inducing morphological changes. Two-photon imaging analyses have revealed that microglia incorporating micronuclei tend to slowly retract their processes. Loss of the cGAS gene alleviates effects on micronucleus-dependent morphological changes. Neuronal micronuclei-harboring microglia also exhibit unique transcriptome signatures. These results demonstrate that neuronal micronuclei serve as niche signals that transform microglia, and provide a potential mechanism for regulation of microglial characteristics in the early-postnatal neocortex.